As a detector for a gas chromatograph, various types of detectors have been practically applied, such as a thermal conductivity detector (TCD), electron capture detector (ECD), flame ionization detector (FID), flame photometric detector (FPD), and flame thermionic detector (FTD). Among these detectors, the FID is most widely used, particularly for the purpose of detecting organic substances. The FID is a device that ionizes sample components in a sample gas by hydrogen flame and detects the resultant ion current. It can attain a wide dynamic range of approximately six orders of magnitude. However, the FID has the following drawbacks: (1) Its ionization efficiency is low, so that its minimum detectable amount is not sufficiently low. (2) Its ionization efficiency for alcohols, aromatic substances, and chlorine substances is low. (3) It requires hydrogen, which is a highly hazardous substance; therefore, an explosion-proof apparatus or similar kind of special equipment must be provided, which makes the entire system more difficult to operate.
On the other hand, as a detector capable of high-sensitivity detection of various compounds from inorganic substances to low-boiling organic compounds, a pulsed discharge detector (PDD) has conventionally been known (for example, refer to U.S. Pat. No. 5,394,092). In the PDD, the molecules or atoms of helium or another substance are excited by a high-voltage pulsed discharge. When those molecules return from the excited state to the ground state, they emit the light with high optical energy. This optical energy is utilized to ionize a molecule or atom to be analyzed, and an ion current brought by the generated ions is detected to obtain a detection signal corresponding to the amount (concentration) of the molecule to be analyzed.
In most cases, the PDD can attain higher ionization efficiencies than the FID. For example, the ionization efficiency of the FID for propane is no higher than 0.0005%, whereas the PDD can achieve a level as high as 0.07%. Despite this advantage, the dynamic range of the PDD is not as wide as that of the FID; the fact is that the former is one or more digits lower than the latter. This is one of the reasons why the PDD is not as widely used as the FID.
The most probable constraining factors for the dynamic range of the conventional PDD are the unstableness of the plasma created for the ionization and the periodic fluctuation of the plasma state. To solve this problem, a discharge ionization current detector has been proposed (for example, refer to U.S. Pat. No. 5,892,364). This detector uses a low-frequency Alternating-Current (AC)-excited dielectric barrier discharge (which is hereinafter referred to as the low-frequency barrier discharge) to create a stable and steady state of plasma. The plasma created by the low-frequency barrier discharge is non-equilibrium atmospheric pressure plasma, which gas temperature does not become hot as easily as the plasma created by the radio-frequency discharge. Furthermore, the periodic fluctuation of the plasma, which occurs due to the transition of the voltage application state if the plasma is created by the pulsed high-voltage excitation, is prevented, so that a stable and steady state of plasma can be easily obtained. Based on these findings, the present inventors have conducted various kinds of research on the discharge ionization current detector using a low-frequency barrier discharge and have made many proposals on this technique (for example, refer to the following documents: International Publication No. WO2009/119050, Shinada et al., “Taikiatsu Maikuro-purazuma Wo Mochiita Gasu Kuromatogurafu You Ion-ka Denryuu Kenshutsuki (Excited Ionization Current Detector for Gas Chromatography by Atmospheric Pressure Microplasma)”, Extended Abstracts of 55th Meeting of Japan Society of Applied Physics and Related Societies in 2008 Spring; and Shinada et al., “Taikiatsu Maikuro-purazuma Wo Mochiita Gasu Kuromatogurafu You Ion-ka Denryuu Kenshutsuki (II) (Excited Ionization Current Detector for Gas Chromatography by Atmospheric Pressure Microplasma: Part II)”, Extended Abstracts of 69th Meeting of Japan Society of Applied Physics in 2008 Autumn).
As explained previously, the low-frequency barrier discharge creates a stable plasma state and is also advantageous for noise reduction. Therefore, the discharge ionization current detector using the low-frequency barrier discharge can attain a high S/N ratio. With respect to its ionization efficiency, although it can attain higher ionization efficiency than that of the FID, its ionization efficiency is equal to or lower than 0.1% at a maximum at present. Accordingly, an ionization current noise corresponding to a required detection limit (a level as high as 1 pgC/sec) is on the order equal to or lower than 1 pA. The implementation thereof requires a sufficient suppression of an influence of a disturbance noise (such as an electromagnetic noise suddenly appearing in a signal cable, or a noise caused by thereto-electromotive force due to a temperature difference) caused by a measurement system. However, it is practically impossible to completely prevent the invasion of a noise from certain parts of the device, such as an opening for introducing and/or discharging a sample gas or a carrier gas. Furthermore, the detection cell is heated up to approximately 400 degrees Centigrade for the detection of a high-boiling component. Therefore, it is very difficult to completely suppress the influence of the thermo-electromotive force occurring between the heated detector cell and a circuit at room temperature.